Slag is the by-prouct of smelting ore to purify metals. They can be considered to be a mixture of metal oxides; however, they can contain metal sulfides (see also matte) and metal atoms in the elemental form. While slags are generally used as a waste removal mechanism in metal smelting, they can also serve other purposes, such as assisting in smelt temperature control and minimizing re-oxidation of the final liquid metal product before casting.